It is known in the prior art to locate one or more blowing strips disposed with their axes parallel to one another, before or after the printing zone above an impression cylinder such that, disregarding the blowing air, the sheet is spread smoothly substantially without contact on the impression cylinder surface. Blowing means of this kind are satisfactory for first-printing. However, they cannot improve the in-register removal of a turned sheet from a previous impression cylinder by means of suckers or grippers or the like disposed on the sheet-turning cylinder and they are also unsuitable for providing a crease-free transfer of the turned sheet to the next printing unit for perfecting printing.
It is known from a number of prior publications, for the sheet to be pressed at the point of tangency on to the surface of the impression cylinder preceding the sheet-turning cylinder by mechanical means, such as guide segments disposed on the periphery of the latter cylinder (cf. DE-AS No. 2,040,712) or by inflatable air hoses (cf. JP-PA No. 58 145450). To some extent these features may provide for in-register removal of the sheet from the impression cylinder by way of the sheet rear edge and a crease-free transfer of the turned sheet to the next printing unit. However, a disadvantage of these systems is that, if no pressure-free corridors are provided, these mechanical means always blur or impair the print image.